Magnetic properties, oxygen content and metal valences in BaRE(Cu_0.5Fe_0.5)₂O_5+δ with RE = Lu, Yb, Y, Eu, Sm, Nd and Pr

Oxygen-deficient double-perovskite samples of BaRE(Cu_0.5Fe_0.5)₂O_5+δ (RE = Lu, Yb, Y, Eu, Sm, Nd and Pr; 0 ≤ δ ≤ 0.45) have been characterized for their magnetic properties. For the samples with RE = Lu-Sm, the oxygen content could be raised above δ ≈ 0 only by means of high-pressure heat treatment. In the case of the larger rare earth elements, i.e. Nd and Pr, excess oxygen was easily introduced under normal pressure as well. The (Cu_0.5Fe_0.5)O₂ planes were found to exhibit antiferromagnetism at or below room temperature and weak spontaneous magnetization below approx. 80 K. The Neel temperature decreases with increasing amount of excess oxygen. Based on the charge-neutrality principle and the ⁵⁷Fe Mossbauer data, the average valences of Fe and Cu were deduced. The ratio of six-coordinated Fe³⁺ to Fe⁵⁺ depends on both the choice of RE and the sample preparation process. The excess oxygen introduced into the RE layer upon the high-pressure heat treatment increases not only the valence of Fe but also the valence of Cu. The behavior of an observed susceptibility anomaly around 80 K was studied upon varying δ.